1. Field of the Invention
The invention relates to a working machine for material webs, particularly a paper format cross-cutter, with two machine parts rotatable in opposition and in particular knife shafts with knives on the circumference forming at least one knife set, the rotary machine parts being synchronized with one another by at least one gear pair having at least one gear body and a tooth system.
2. Description of Related Art
The term size or format cross-cutters is normally used for cross-cutters, which cut from material webs divided up into strips by different formats, e.g. the standard DIN A 3 of DIN A 4 formats. Normally two or three knives are located on each knife shaft and the knife shafts are so associated with one another that a cutting engagement is in each case produced between two knives set in matching manner.
Here and hereinafter the term cutting engagement is used to mean as association of the two knives which is such that there is a perfect cut between them. The knives need not and should not be in contact with one another.
The so-called synchronous length of cut between two cuts is determined by the circumferential distance between knives on the particular knife shaft, because it is necessary that during the cutting engagement the web speed and the peripheral speed of the knives coincide. Thus, only one format length can be cut during a uniform rotation of the knife shafts.
In order to allow a change to the format length, several knife shafts were provided for each machine, which had to be changed on modifying the format and required realignment with respect to the knife setting. Such a reequipping was very time and labour consuming and required a long machine shutdown.
Another possibility for changing the format length is known from DE-A-3,608,111, corresponding to U.S. Pat. No. 4,881,436. In this case the knife shafts are driven directly by direct current motors, which allow very precise and rapid speed changes and optionally also several times per shaft rotation. As a result of this drive with varying velocity ratio it is possible to delay the knife shafts, e.g. between two cutting engagement positions, whilst the material web continues to run at a uniform speed. As a result, the length of cut increases and diverges from the synchronous format. Such periodic decelerations and accelerations of the knife shafts were previously also brought about using mechanical drives with varying velocity ratios. This solution with a drive having a varying velocity ratio is very flexible with respect to the lengths to be set, but requires a reduction in the maximum performance characteristics of the cross-cutter, in order to be able to keep within reasonable limits the accelerations and decelerations which occur. Thus, e.g. with a synchronous length of cut of 325 mm in the case of a reduction of the length of cut to 280 mm by a drive with varying velocity ratio, the machine speed must be reduced from 350 to 150 m/min, i.e. more than halved.
DE-A-3,608,111 discloses clamp collars by means of which the synchronization gears can be fixed to the knife shafts. These clamp collars have a rotating gap filled with a pressure medium. As a result of the rotating gap a certain radial elasticity existed making it possible to set the somewhat eccentrically manufactured gear at one point of the circumference to a minimum backlash. The radial elasticity prevented a seizing of the gears.
Backlash is defined by Webster's Universal Unabridged Dictionary, 1936 Edition, as the backward surge of a pair or a train of toothed wheels acting under a variable driving pressure; the loose play of such a part. Backlash equals the clearance allowance between the flanks of the teeth in gear.
U.S. Pat. No. 3,182,540 discloses a paper cross-cutter, which carries on a knife shaft numerous knives, which cooperate with a roller-like counterroll for performing a squeezing cut. On it surface the counterroll has raised and lowered areas, so that through a specific reciprocal configuration of knife shaft and counterroll only certain knives perform the squeezing cut in each case, so that a different format length can be set.
It is also known from German Patent 602,721 to engage for cutting purposes only two knife shafts equipped in each case with knives by means of a hand lever. As a result of a cam control they are forced apart again subsequently.
DE-A-4,005,028 describes two knife shafts equipped with knives, which use the opposing knife shaft as an anvil roller for performing a squeezing knife cut. As a result of a corresponding knife configuration alternately longer and shorter portions (intermediate strips) can be cut out.